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Abstract

Vertical junction resonant microdisk modulators and switches have been demonstrated with exceptionally low power consumption, low-voltage operation, high-speed, and compact size. This paper reviews the progress of vertical junction microdisk modulators, provides detailed design data, and compares vertical junction performance to lateral junction performance. The use of a vertical junction maximizes the overlap of the depletion region with the optical mode thereby minimizing both the drive voltage and power consumption of a depletion-mode modulator. Further, the vertical junction enables contact to be made from the interior of the resonator and therein a hard outer wall to be formed that minimizes radiation in small diameter resonators, further reducing the capacitance and drive power of the modulator. Initial simple vertical junction modulators using depletion-mode operation demonstrated the first sub-100fJ/bit silicon modulators. With more intricate doping schemes and through the use of AC-coupled drive signals, 3.5μm diameter vertical junction microdisk modulators have recently achieved a communications efficiency of 3fJ/bit, making these modulators the smallest and lowest power modulators demonstrated to date, in any material system. Additionally, the demonstration was performed at 12.5Gb/s, required a peak-to-peak signal level of only 1V, and achieved bit-error-rates below 10−12 without requiring signal pre-emphasis. As an additional benefit to the use of interior contacts, higher-order active filters can be constructed from multiple vertical-junction modulators without interference of the electrodes. Doing so, we demonstrated second-order active high-speed bandpass switches with ~2.5ns switching speeds, and power penalties of only 0.4dB. Through the use of vertical junctions in resonant modulators, we have achieved the lowest power consumption, lowest voltage, and smallest silicon modulators demonstrated to date.

Figures (7)

(a) The change in the real and imaginary components of the refractive index are plotted as a function of carrier concentration for both electrons and holes at a wavelength of λ = 1550nm. The change in the real part of the refractive index is larger than that of the imaginary part and impacted more greatly by holes. The plots were obtained from curve fits to the experimental data in [23]. (b) A comparison of horizontal versus vertical junction modulators based on the depletion approximation. A plot of the fractional change in the waveguide depletion obtained from the depletion approximation going from a 0V to 2.5V applied for 0.25um and 0.5um silicon waveguides. The fractional change in depletion is more than a factor of 2 larger for the narrow guide with a vertical junction.

(a) A diagram of our vertical junction microdisk modulator, (b) a cross-section view of the lowest order TE mode of the microdisk modulator and how it overlaps the depletion region, (c) Finite Element Model (FEM) results of the carrier distribution as a function of applied voltage, and (d) the calculated optical response of the modulator as a function of applied voltage obtained by inserting the carrier distribution (c) into the mode-solver to obtain the resulting mode (b), showing quality factor and frequency shift.

(a) Diagram of a more advanced partially-doped microdisk modulator with a more intricate doping scheme designed to achieve lower capacitance and lower energy consumption, and (b) a plot of the measured frequency shifts of a partial (i.e., half) and fully doped ring as a function of applied bias. Importantly, the slope of the frequency shift curve gets steeper towards forward bias.

(a) TDR measurements of the instantaneous power consumption and switching energy for a 1V AC-coupled drive, (b) a 12.5Gb/s eye-diagram for a 1V AC-coupled drive signal, (c) TDR measurements of the instantaneous power consumption and switching energy for a 1.5V AC-coupled drive, and (d) a 12.5Gb/s eye-diagram for a 1.5V AC-coupled drive signal.

A diagram (a) and a scanning electron micrograph (b) of a 2nd order microdisk bandpass switch formed from a pair of coupled microdisk modulators. The switch operates by applying a forward bias across the p-n junction. The response of short (1501nm) and long wavelength (1533nm) bandpasses, separated by a free-spectral-range (FSR) are depicted in (c) and (d), respectively. With an applied bias of only 1.09V/1mA, the bandpasses are shifted fully out of the channel. The filter responses have 1dB bandwidths of 33GHz and 46GHz, respectively.

A 10Gb/s NRZ PRBS was generated by an external lithium niobate modulator and sent through the 1533nm bandpass of the switch depicted in Fig. 3. The switch was then activated with a square-wave modulation. The outputs of both the Thru (red) and Drop (blue) ports are shown in (a). Extinctions of −16dB and −20dB are achieved in the Thru and Drop ports, respectively. (b) Eye diagrams of the 10Gb/s PRBS were obtained in the Thru and Drop ports under switch activation. No perceptible degradation in the eye diagram was observed in either port. (c) The bit-error-rate (BER) of the switch as a function of received power in the static states of the Thru and Drop ports were compared to that of the off-resonance Thru port. A power penalty close to 0dB was observed in the Thru port and a power penalty of only −0.4dB was observed in the Drop port at a BER of <10−12.